A variety of processes involve the use of webs that are wound, unwound and/or rewound during the performance of the process. Movement of the web through apparatus performing the process can have frictional contact between the web and rotating members or stationary members and guide devices across which the web is directed. Consequently, the web can accumulate both positive and negative static charges. Some webs such as, for example, paper webs accept and hold static charges readily. Build up of static charges in the web can impact equipment or process performance and functionality. Charges in the web can cause the web to be attracted to or repelled from transport surfaces, interfering with proper transport and direction of the web through the process equipment.
Electrostatic charges can present significant hazards to operator safety, product quality and electronic process control. If the charge level on the roll or web reaches a critical limit, a spark can occur, arcing to conductive things that are near by. Damage can occur to critical electronic components, and personnel in proximity can be injured. Even apart from the potential danger to individuals, the danger and damage to equipment can be significant and costly to repair.
In some converting operations, a large parent roll of the web is unwound, processed and rewound into another roll or rolls. For example, a simple slitting operation may be performed to divide a wide web into a plurality of narrower webs. Plastic films, coated paper and other materials rewound in such slitting and rewinding converting operations can become large capacitors, capable of storing in excess of 50,000 volts of static electricity. These highly charged capacitors can discharge, causing shocks to operators potentially resulting in burns or subsequent injuries due to the physical reaction from receiving an electrical shock. Rapid discharge in the nature of arcing to nearby conductors can producing a strong arc several inches long, causing a potential fire or explosion hazard under some conditions. For example, the paper dust present from a slitting operation potentially can be explosive.
Build-up of static charges also can cause damage to the web. If the electrostatic charge is sufficiently high, a spontaneous discharge in the form of sparks can occur. Products with coated surfaces, such as silicone release liners, are easily damaged by such sparks, which can result in significant product scrap rates. Highly cosmetic materials may develop visible streaking when damaged by spontaneous discharge sparks, referred to as “static trees”, also resulting in lost production yield.
The web also can become contaminated as the result of building static electric charges. Statically charged objects, such as wound rolls, will attract and retain contamination such as slitter dust, atmospheric contaminants and debris or chips from machining. Such contamination wound into a roll damages not only the immediate area contacted by the contaminant, but also can damage layers in the roll above and below the contaminant by puckering, wrinkling or other layer distortion.
Further, static discharges and the associated electromagnetic interference can damage sensitive electronic equipment, causing loss of data memory and the generation of false signals. Static discharges also can cause programmable logic controllers, computers and other electronic control devices to lock-up or freeze while equipment is operating.
Difficulties have been encountered in designing and applying static charge neutralizing devices on equipment in which a web is wound or unwound in a roll. The changing diameter of the roll has made it difficult and/or costly to effectively neutralize static charges accumulating therein.
Several devices have been used, with differing degrees of success. High output static neutralizing equipment has been mounted in fixed positions relative to the building or unwinding roll, to work over the continuous changing distance as the roll diameter changes. Such devices have become less effective as the distance between the outside surface of the roll and the static neutralizing equipment increases.
In another known design, a rider roll is used in direct contact with the material winding on or unwinding from the supply roll. The rider roll rides the top of the outer surface as the diameter of the supply roll changes. Static neutralizing equipment is mounted on the rider roll support arms so that it maintains an appropriate working distance from the web roll outer surface as the diameter changes. Uses of such devices are limited to material and processes that can tolerate direct contact of the rider roll on the roll of material.
It is known also to use ionizing blowers in fixed locations to propel a stream of ionized air over a continuously changing distance as the roll diameter changes. These devices have been limited to slower web speeds, generally no greater than 300 to 500 feet per minute, and become less effective as the distance increases between the outside surface of the roll and the ionizing air blower.
What is needed in the art is a static neutralizing device that effectively follows the moving surface of a building or unwinding roll, to effectively neutralize static charges even as the roll diameter changes.